<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <label for="province">省份：</label>
    <select id="province">
        <option value="">--请选择省份--</option>
    </select>

    <label for="city">城市：</label>
    <select id="city">
        <option value="">--请选择城市--</option>
    </select>

    <label for="town">镇街：</label>
    <select id="town">
        <option value="">--请选择镇街--</option>
    </select>

    <script>
        /*
        *  ajax 四部曲
        *  1. 创建ajax对象
        *
        *  省市镇街联动
        */
        let province = document.getElementById('province');
        let city = document.getElementById('city');
        let town = document.getElementById('town')
        let xmlHttp = new XMLHttpRequest();
    </script>

</body>
</html>